Proton circuits within biological membranes, the foundation of natural bioenergetic systems, are significantly influenced by the lipid compositions of different biological membranes. In this study, we investigate the influence of mixed lipid membrane composition on the proton transfer (PT) properties on the surface of the membrane. We track the excited-state PT (ESPT) process from a tethered probe to the membrane with timescales and length scales of PT relevant to bioenergetic systems. Two processes can happen during ESPT: the initial PT from the probe to the membrane at short timescales, followed by diffusion of dissociated protons around the probe on the membrane, and the possible geminate recombination with the probe at longer timescales. Here, we use membranes composed of mixtures of phosphatidylcholine (PC) and phosphatidic acid (PA). We show that the changes in the ESPT properties are not monotonous with the concentration of the lipid mixture; at a low concentration of PA in PC, we find that the membrane is a poor proton acceptor. Molecular dynamics simulations indicate that the membrane is more structured at this specific lipid mixture, with the least number of defects. Accordingly, we suggest that the structure of the membrane is an important factor in facilitating PT. We further show that the composition of the membrane affects the geminate proton diffusion around the probe, whereas, on a timescale of tens of nanoseconds, the dissociated proton is mostly lateral restricted to the membrane plane in PA membranes, while in PC, the diffusion is less restricted by the membrane.

中文翻译：

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混合脂质膜表面的质子扩散凸显了膜成分的作用


生物膜内的质子回路是天然生物能系统的基础，受到不同生物膜的脂质成分的显着影响。在这项研究中，我们研究了混合脂质膜成分对膜表面质子转移（PT）特性的影响。我们跟踪从系留探针到膜的激发态 PT (ESPT) 过程，并使用与生物能系统相关的 PT 时间尺度和长度尺度。 ESPT 期间可能发生两个过程：在短时间内从探针到膜的初始 PT，随后解离质子在膜上探针周围的扩散，以及在较长时间尺度下可能与探针的成对重组。在这里，我们使用由磷脂酰胆碱 (PC) 和磷脂酸 (PA) 混合物组成的膜。我们发现 ESPT 特性的变化并不随着脂质混合物的浓度而单调变化；当 PC 中的 PA 浓度较低时，我们发现该膜是一个较差的质子受体。分子动力学模拟表明，在这种特定的脂质混合物中，膜的结构更加结构化，缺陷数量最少。因此，我们认为膜的结构是促进 PT 的重要因素。我们进一步表明，膜的成分影响探针周围的成对质子扩散，而在数十纳秒的时间尺度上，PA膜中解离的质子大部分横向限制在膜平面上，而在PC中，扩散是受膜的限制较小。